Considerable efforts have been made in the investigation for the development of aluminum-based materials of high resistance, which would be able to satisfy the demands of advanced designs of the electrical industry.
The metallic matrix aluminum-based composites are under consideration due to the possibility that the addition of reinforcing particles, added to aluminum or its alloys, offers to improve the properties of aluminum and their alloys with respect to the elastic module, yield stress, maximum stress, and deformation.
In a general way, two main types of hardened aluminum-based alloys exist with particles as a second phase; one involves a heat treatment, while the other does not. In thermo handle alloys, fine particles of a second phase, commonly referred to as precipitated, produce products with high resistance and rigidity. This occurs when a solid supersaturated solution precipitates the excess of solute. This process is favored in alloy systems that present an increase in the solubility of the solute as the temperature is increasing too, in comparison with low or room temperature.
On the other hand, the hardening by dispersion in aluminum alloys non-thermo handle is based on the production of a fine distribution of incoherent particles of the second phase that hardens the aluminum matrix or its alloys preventing the movement of dislocations (plastic flow) due to their near spacing. In this case, the particles of the second phase have a small or null solubility in the solid state even at high temperature. This type of reaction is commonly referred to as hardening by dispersion. In both types of alloys it is desirable to maintain reinforcement particles of the secondary phase in a fine size (0.1-0.5 microns) and near spacing to obtain a good combination of hardness and resistance (Principios de Metalurgia Fisica, Roberto And Reed-Hill, Editorial CECSA, Chapter 9, page 323, 1980). These materials are produced by a heat treatment, which involves the dissolution of particles of the second phase in the metallic matrix, followed by the steps of solution and aging, which provides a fine distribution of precipitates of the secondary phase. Said precipitated, affects the resistance of the structure by restoration of tensions inside the metallic matrix.
It has been demonstrated, by theoretical and experimental considerations (International Journal of Materials and Product Technology, Vol 15, Nos. 3/4/5, pp 356-408, Dispersion-Strengthened Aluminum Prepared by Mechanical Alloying by M. Besterci), that the maximum effect of hardening is obtained by means of the following structural parameters:                a) The particle size of the reinforcing particles of the secondary phase (dispersoids) should not exceed 50 nm. The particles of larger size are of little significance from the point of view of the hardening.        b) The average distance among the reinforcement particles must be within the range of 100 to 500 nm and its distribution should be uniform, without presence of heterogeneity and conglomerates.        
The Metallic Matrix Composites (MMC) are at the present, potential candidates for a great variety of applications, for example; structural applications, in the automotive industry and in the industry of the electrical conductors to mention some of them. Diverse techniques for the production of MMC can be used; these can be grouped into two main types depending on the state of the matrix during the manufacture process: solid or liquid route. Although the production of MMC by liquid metal processing is receiving great attention due to its relatively low cost, frequently the following disadvantages are presented:
i) heterogeneous distribution of particles of ceramics or fibers, due to the agglomeration and dendrite segregation that is a variation in concentrations on microscopic scale due to the different solidification points which present the elements or composites of an alloy, and that is generated during the solidification of alloys; and
ii) undesirable chemical reactions in the inter-phase due to the high temperature of the fused material.
The liquid route is covered by conventional techniques of metallurgy (fusion and cast); the solid route is covered by Powder Metallurgy (PM). Additionally, the PM could include the processing of the initial materials by Mechanical Alloying (MA) and Mechanical Milling (MM). Also, new and novel materials can be produced by MA and MM, including metallic and ceramic. The MA can be defined as the dry milling of two or more elements or composites in a mill for obtaining nanometric materials such as powdered alloys or composites with homogenous multiphase dispersions. This process occurs by means of the repeated welding and fracture of a metallic and non-metallic powdered mixture using mechanically activated balls.
It has been established as the term “mechanical alloying” when pure elements are involved, and “mechanical milling” when powdered pre-alloyed are involved. The MA is a method to produce metallic powdered composites with a controlled fine microstructure and uniformity. This occurs by the phenomenon of fracture and subsequent welding of a powdered particle mixture during the milling with impact of high energy in a controlled environment, for example, in a ball type mill, in the presence of process control agent (optional) that prevents the agglomeration of particles. During the processing, the oxide shell generally present in the surface of the powdered particles is incorporated inside the particle of the composite and homogenously dispersed. In a similar way, the metallic and non-metallic components are also finely dispersed inside the powdered particles. The materials produced by MA are later consolidated by diverse routes, for example: sintered pressing, extrusion, rolling, cast, etc., only to mention some of them.
The MA and MM processes have been widely recognized as alternative routes in the formation of metastable phases for select applications. The MA and MM processes are novel techniques to produce advanced materials with unusual properties, due to the microstructure refinement, even in immiscible systems. In this way, combining PM with MA or MM, a new generation of materials can be obtained.
The necessity to increase the mechanical properties in aluminum alloy has motivated the study of the Composite Materials (CM) aluminum based. These materials are required due to their low density and high specific stiffness. In addition, the reinforcing ceramic particles significantly increase the wear resistance. By means of PM methods, it is possible to produce unique CM with an extremely fine distribution of particles of reinforcement, that otherwise would not be possible to obtain by means of the conventional techniques of metal and alloys molding.
The CM are usually produced when mixing non-metallic reinforcing particles such as sands, powder, fibers, or similar within a metallic matrix. For example, MC aluminum-based, are basically formed from aluminum or of a commercial aluminum alloy (namely 1350, 6061, 2024, A356, etc.) reinforced with ceramic particles such as silicon carbide or powder of aluminum oxide.
In spite of the growing market, the high manufacture cost of the composite materials (CM) has limited their availability to be competitively evaluated with the non-reinforced metallic materials. Traditionally, the manufacture of CM, has employed techniques related to the Powder Metallurgy (PM), such as the compaction of particle mixtures or ceramic fibers and powdered aluminum. Unfortunately, the high cost of powdered metals, pyrophoric risks, and burst associated with the handling of great amounts of powder, have avoid a significant reduction in the cost of CM produced by these routes. Additionally, the use of liquid metal in the production of CM has been limited to the infiltration in ceramic preform. Similarly, the mixture of ceramic materials inside liquid Aluminum using agitation methods, has not been advantageous due to problems of impregnation of the liquid in the fine particles (wettabillity of particles), which have a great surface area, joining the quick oxidation of the liquid aluminum, highly reactive, during the agitation.
There are several methods or variants in the process of agitation in liquid state, for example compo casting and others more sophisticated, such as the one reported in the U.S. Pat. No. 6,491,423 entitled “Apparatus for Mixing Particles into Liquid Medium” by M. D. Skibo and D. M. Schuster. However, the main problem still continues to be the incomplete, low or null impregnating (wettabillity) of the reinforcing particles. By means of the present invention, the impregnation is increased considerably and the incorporation of the reinforcing particles inside the aluminum matrix or its alloys is facilitated. Also, the oxidation of reinforcing particles is prevented by the preparation or pre-conditioning in which they are submitted, as explained further on.
The main problem with many conventional aluminum alloys is, when the requirements of density and mechanical resistance, are reaching they are not enough ductile to be usable. Also, these aluminum alloys of high mechanical resistance do not fulfill the minimum requirements of conductivity to be able to be used in electrical applications.
The aluminum-base alloy obtained according to the process of the present invention presents high mechanical resistance and at the same time maintains the aluminum intrinsic properties, for example density, % of elongation (ductility) and electrical conductivity.